The effects of milbemycin A(4) (MB A(4)), milbemycin oxime A(4) (MBO A(4)) and milbemycin beta(1) (MB beta(1)) on reversing multidrug resistance (MDR) of tumor cells were firstly conducted according to the following research, including MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay, the accumulation of adriamycin, the accumulation and efflux of rhodamine 123 (Rh123), the regulations of MDR1 gene, and expression of P-gp. The three milbemycins (5muM) showed strong potency to increase adriamycin cytotoxicity toward adriamycin-resistant human breast carcinoma cells MCF-7/adr with reversal fold (RF) of 21.42, 19.06 and 14.89, respectively. In addition, the mechanisms of milbemycins on P-glycoprotein (P-gp)-mediated MDR demonstrated that the milbemycins significantly increased the intracellular accumulations of adriamycin and Rh123 via inhibiting P-gp transport function. Based on the analysis of the P-gp and MDR1 gene expression using flow cytometry and RT-PCR, the results revealed that milbemycin compounds, particularly MB A(4), could regulate down the expression of the P-gp and MDR1 gene. These findings suggest that the milbemycins probably represent promising agents for overcoming MDR in cancer therapy, and especially MB A(4) is better modulator with the lowest toxicity.